19. Incorrect. The answer is false, not true. There is an important
theorem in Statistics which links the normal, the Chi-square, and the t-student
distribution function which establishes that the ratio of a random variable 'Z'
which has a standardized normal distribution and the square root of another
random variable 'Y' which has a Chi-square distribution with 'r' degrees of freedom
follows a t-student distribution function with 'r' degrees of freedom, provided
that Z and Y are independent.

In our case, the standardized
diameter is Ds = (D - 2)/0.5 ~ N(0,1), and
K ~ χ2(20) because the mean of the thickness is equal to the
number of degree of freedom of the Chi-square distribution. Therefore, we can
define 'L' as follows: